Download Optimize Power Factor Correction Inductors

designfeature
CHARLES WILD, Precision, Inc.
Optimize Power Factor
Correction Inductors
The world is moving toward
greater use of power factor correction (PFC). This
requires attention to the components employed for power
factor correction, particularly
the associated inductor.
E
xcessive use of electricity and poor efficiencies are no longer acceptable. Eliminating poor power factor will result in increasing electrical efficiencies and will reduce the overall use of electricity. Until
now, most inductors used for Power Factor Correction have been
custom designed for their application, resulting in higher costs and
longer lead times. Using online design tools and properly grouping
circuit parameters, standard inductor products can be used to solve
power factor issues.
Eliminating power factor issues can be handled in many different ways.
However, not all power factor corrections work for every power factor issue.
Without properly understanding what power factor is, and what affects it, can lead
to even more inefficient designs.
POWER FACTOR
Power factor is a measure of how efficiently, or inefficiently, electrical power is
used. Power factor is expressed as a fraction between 0 and 1 and is the ratio
between real power and apparent power.
Power Factor =
Real Power
Apparent Power
Apparent power is a function of the total impedance (Z) of the circuit, and is the
vector sum of real and reactive power. Fig. 1 shows the vector relationships.
To further define the two
components of apparent power,
Apparent Power (VA)
the real power consists of cirReactive
cuit’s dissipative elements, usuPower
VAR
ally resistances (R) while reactive
power consists of a circuit’s reacReal Power (VA)
tive elements (X) (capacitors and
coils). Since apparent power is
the product of a circuit’s voltage Fig. 1. Vector relationships for ac power
and current, without reference to
Table 1. Guidelines for magnetic core selection
Performance
DC Bias
(Saturation)
AC
Losses
Lowest
fig
1
Cost
Soft
Saturation
Thermal
Stability
Best
High Flux
Gapped
Ferrite
Gapped Ferrite
High Flux
Kool Mu
2nd Best
XFlux
MPP
Iron Powder
Kool Mu
High Flux
3rd Best
Laminated
Silicon Iron
Laminated
Amorphous
Kool Mu
MPP
MPP
Lowest
Laminated
Amorphous
Kool Mu
XFlux
Iron Powder
18 Power Electronics Technology | April 2011
www.powerelectronics.com
INDUCTOR VARIATIONS
An inductor or reactor is a passive electrical component
that can store energy in a magnetic field created by the
electric current passing through it. An inductor’s ability
to store magnetic energy is measured by its inductance,
in units of henries. Typically, an inductor is a conducting
wire shaped as a coil; the loops help to create a strong
magnetic field inside the coil due to Ampere’s
Law. Due to the time-varying magnetic field
inside the coil, a voltage is induced, according to Faraday’s law of electromagnetic
induction, which by Lenz’s Law opposes
the change in current that created it.
Inductors are one of the basic components
used in electronics where current and voltage change with time, due to the ability of
inductors to delay and reshape alternating
currents. Inductors called chokes are used as
parts of filters in power supplies, or to block
AC signals from passing through a circuit.
Almost any style of power inductor can
inductor with be used for Power Factor Correction, but
for this analysis, toroidal inductors are used.
Toroidal inductors offer higher performance
than other types of inductors. Toroidal inductors use less
volume, weigh less and emit lower electromagnetic interference (EMI). Winding heat transfer is more efficient as a
proportionally larger surface area of copper is available to
be cooled. The toroidal geometry leads to near complete
magnetic field cancellation outside of its coil, allowing the
toroidal inductor to have less electromagnetic Interference
(EMI) when compared to other inductors of equal power
rating. Single layer winding and a full 360 degree winding
around the core make for excellent turn to turn coupling
and lower leakage inductance.
Power Level Product
Chart 1: Temperature Rise
140
0.18
0.17
0.16
0.15
0.14
0.13
0.12
0.11
0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
130
120
110
100
90
80
70
60
50
Temperature Rise
phase angle, it is measured in Volt-Amps (VA).
Power Factor is a practical measure of the efficiency
of power usage in a circuit. Circuits containing purely
resistive elements have a power factor of 1.0.
Circuits containing inductive or capacitive elements have a power factor below 1.0. For
two circuits utilizing the same amount of
real power, the system with the lower
power factor will have higher circulating currents due to energy that returns
to the source from energy storage in
the load. These higher currents produce
higher losses and reduce overall efficiency.
A lower power factor circuit will have a
higher apparent power and higher losses for
the same amount of real power.
Power factor correction adjusts the power
factor in an AC power circuit closer to unity Fig. 2. Toroidal
by adding an equal and opposite load to the improved mounting
circuit to cancel out the effects of the load’s
reactance. Capacitors and inductors can be added which
act to cancel the inductive or capacitive effects of the
load. Capacitive reactance can only be canceled by adding
inductive reactance; inductive reactance can only be cancelled by adding capacitive reactance. The effect of the
opposing reactances is to bring the circuit’s total impedance equal to its total resistance. In other words, the real
power and the apparent power match, making the power
factor closer (or equal) to 1.
Another reason to resolve Power Factor issues is to
comply with international regulations, especially if you
intend to sell your equipment in Europe. The European
Union (EU) established limits on harmonic currents that
can appear on the mains (AC line) of switch-mode power
supplies. EN61000-3-2 applies to power supplies with
input power of 75 watts or greater, and that pull up to 16
amps off the mains. Power supplies with PFC circuits that
meet EN61000-3-2 inherently have high power factors
that are typically 0.97 or better.
IC manufacturers have been designing their circuitry to
meet the European regulations, and are anticipating that
similar regulations will be developed for other parts of the
world. In the design of their circuitry, most IC manufacturers have initiated many inductor designs to meet the
requirements of their circuitry. These designs have typically been “off the shelf” inductors, not intended to meet
the demands of Power Factor Correction or expensive,
highly custom designed inductors.
40
PL Product
Max Pout
Ripple %
PL Product Min
30
20
10
0
100
200
300
400
500
Power Level in Watts
Fig . 3. PFC inductor characteristics
fig 3
www.powerelectronics.com April 2011 | Power Electronics Technology
19
INDUCTORperformance
Power Level Product
20 Power Electronics Technology | April 2011
Ripple (percent)
permeability material, consideration
Toroidal inductors can be used in any
Table 2. Frequency
has to be given to where it is placed
inductor application that can accomrange of magnetic
and what are its losses. These considermodate its shape. Although usable, tormaterials
ations argue in favor of distributed gap
oidal inductors are not always practical
Core Material
Frequency Range
materials such as powdered iron, MPP,
for some applications.
High Flux
Up to 50 kHz
sendust, KoolMu, and against ferrite.
Toroidal inductor cores are availMPP
Up to 200 kHz
Mounting a toroidal inductor has
able in many materials: silicon steel,
Kool Mu
Up to 200 kHz
become easier as the industry has stannickel iron, moly-permalloy powder,
dardized on carriers and landing patiron powdered, amorphous, ferrites,
Powdered Iron
Up to 25 kHz
terns. Depending on the toroid size, it
and others. Silicon steel and nickel iron
Laminated Steel
Up to 10 kHz
may be further secured using epoxy,
are available as tape wound cores or
Gapped Ferrite
20 Hz to 2 MHz
mounting screws, or even tie wraps.
laminated pieces. Non-magnetic torAmorphous Strip
10 kHz to 100 kHz
Horizontal and vertical mounting
oids are also available to make air
XFlux
Up to 25 kHz
options are available to the circuit
core toroidal inductors. For Power
designer, depending on the available board (or enclosure)
Factor Correction applications, powdered iron, Kool-mu
mounting area. For example, placing a toroid flat on the
(Sendust), Hi Flux, moly-permalloy and XFlux each
board may take up too much board area, while in some
provide their own unique solutions to frequency, power
applications standing the toroid vertically will interfere
handling and biasing needs. Table 1 shows general “rule of
with a height restriction. Fig. 2 shows a toroidal inductor
thumb” guidelines and tradeoffs for material selection.
with improved mounting characteristics.
Another useful criterion for material selection is the
One reason PFC circuits can be difficult to design is
frequency range the material is intended for. Operating
that engineers must fully understand the PFC inductor’s
frequency decisions involve tradeoffs in core & winding
operating characteristics and its effect on the PFC cirlosses of the inductor With wound magnetic components
cuit as design criteria changes. The inductor selection is
and commonly available core materials, a study of these
still extremely custom based upon the circuit designer’s
tradeoffs leads to the conclusion that operation up to
choices of operating mode, frequency range, maximum
300-400 kHz is possible but is a diminish in return after
output power, efficiency, max and min AC voltages, outabout 125 kHz. Table 2 gives some general guidelines for
put voltage and ripple current. Typically, the most impormaximum frequency ranges of several materials.
tant factor in the final choice is size and cost, and not
Optimally, the inductor should be small and inexpenthe most optimum. The magnetic designer must balance
sive. However, reliability or performance of the inductor
these requirements, even though most of these parameters
cannot be sacrificed. To avoid sudden saturation, the core
move in opposite directions, working against each other.
will need to be of relatively low permeability to support
Communication between the circuit designer and the
the DC component of operation. If a gap is used in a high
magnetic designer help to prioritize which parameter is
Chart 2: Ripple %
more crucial to satisfy the design requirements.
0.18
–0.7
Based upon customer demand, several companies have
0.17
–0.65
0.16
developed standard inductor designs specifically intended
0.15
–0.6
for power factor correction in power supply design. In
0.14
–0.55
doing so, these companies have standardized around
0.13
–0.5
common operation parameters. One company’s line of
0.12
–0.45
0.11
power factor correction inductors is based upon a CCM
0.1
–0.4
(continuous conduction mode) method of operation, 100
0.09
–0.35
kHz operation, 92% efficiency, VIN of 85-265 and VOUT of
0.08
–0.3
385.
In standardizing the inductor, they have also stream0.07
–0.25
0.06
lined the inductor selection process.
PL Product
0.05
–0.2
Max Pout
During the 2010 APEC, Welly Chou, of Precision, Inc.
0.04
Ripple %
–0.15
presented
his development of the PL Product tool. Chou
PL Product Min
0.03
–0.1
developed an interactive model called PL Product to be
0.02
–0.05
0.01
used by design engineers to evaluate and speed the selec0
0
tion of PFC inductors in the circuitry. Traditionally, PFC
100
200
300
400
500
Power Level in Watts
inductors are developed based on a set of circuit parameters, i.e., minimum line voltage, switching frequency,
ripple current, DC output voltage, etc. While such appliFig. 4. Expected ripple of the selected inductor
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INDUCTORperformance
cation specific approach optimizes the inductor design, it
makes it difficult to understand the relationship and the
trade-offs among the circuit parameters and the inductor.
PL product simplifies this by separating the output power
(P) and the Inductance (L) needed for a desired ripple
current from the PFC circuit parameters, leaving designers with a simple computation to derive the minimum
PL required without delving into the intricacies of the
inductor. Fig. 3 shows the characteristics of the selected
inductor. Anticipated ripple of the selected inductor is
shown in Fig. 4.
To support Chou’s PL Product calculation, Precision,
Inc. has developed an online PL Product calculator
(http://pfc.precision-inc.com/), a user-driven software
used to characterize PFC inductors over a range of output
power. Based on desired circuit operating parameters,
designers can calculate the PL Product required for their
application and correlate it to the PL Curves of various
PFC inductors. This will not only simplify the design
process by making inductor selection easier, but also
allow circuit and PFC inductor characteristics to be realized under various operating conditions. The PL Product
makes it easier for circuit designers to understand the
relationship and trade-offs among circuit parameters and
the inductor.
Low Power Factor continues to be a growing issue. The
European Union has already identified this situation and
has developed regulations to address Power Factor
Correction. As more regulations come forward, power factor correction will become needed in almost all applications. Circuit designers will continue to face difficult
design choices, including cost vs. performance choices in
magnetic components. Using the proper tools and industry
standard inductor designs and footprints is the key to costeffective solutions to circuit designs. Understanding the
proper techniques for correcting power factor will help
increase electrical efficiency.
Many thanks to Lyle Shaw and David Anderson of
Precision, Inc. and Mark Swihart of Magnetics, Inc. in support of this article.
REFERENCES:
1. A
Practical Approach To Boost CCM Power Factor Corrector Design - D.
Michael Shields
2. PL product tool – Welly Chou
3. Wikipedia
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